Skip more under ant -Ddocs.skip.

 - skip docs.lib, which doesn't route through the staged-docs
   macro
 - skip copying docs to the staged maven distribution
 - skip maven local deploy of doc jar under this mode
 - skip devel docs tarring
fix uptodate checking for osgi.done

A new file in ./build/pack/lib/scala-compiler.jar should trigger this task,
otherwise ./build/osgi/org.scala-lang.scala-compiler.jar becomes stale.
Use overwriting copy uniformly in our build.

The staleness problem in our build had its roots in
the flawed uptodate check fixed in the previous commit,
but I spent a lot of time looking at these first. Maybe
one was also complicit.

We've got better things to worry about, so lets make
the all overwrite.